1. Field of the Invention
The present invention relates to an ink tank (liquid tank) for holding ink used in inkjet recording by discharging a liquid such as ink.
2. Description of the Related Art
A typical ink tank includes an ink chamber for directly holding ink and an absorption chamber for holding ink by absorbing the ink with an absorber disposed inside the absorption chamber. The ink chamber and the absorption chamber are connected via a communication hole. In such an ink tank, air is taken in from an air inlet into the absorption chamber and enters the ink chamber via the communication hole while ink in the absorption chamber is being consumed for recording. At the same time, ink is supplied from the ink chamber to the absorption chamber by gas-liquid exchange. A technology enabling high-speed recording by stabilizing and speeding up the gas-liquid exchange and thereby stabilizing the negative pressure inside the ink tank to obtain stable discharge of recording heads is disclosed in Japanese Patent No. 2951818 (corresponding to U.S. Pat. No. 5,509,140).
FIG. 16 illustrates a cross-sectional schematic view of an ink tank disclosed in Japanese Patent Laid-Open No. 2000-33715 (corresponding to U.S. Pat. No. 6,502,931). An ink tank 51 includes an ink chamber 57 for holding ink 56 and an absorption chamber 55 for storing absorbers 54a and 54b. The ink chamber 57 and the absorption chamber 55 are separated by a partition 58 having a communication hole 59.
The ink chamber 57 is sealed except for the communication hole 59. The sidewalls of the absorption chamber 55 have an air inlet 52 for taking in air to the ink tank 51 while ink is being consumed and an ink outlet 53 for supplying ink to a recording head (not shown in the drawing). The ink outlet 53 is formed at the tip of a hollow projection. A pressure contact body 62 is interposed between the ink outlet 53 and the absorber 54b. Inside the absorption chamber 55, the absorbers 54a and 54b are held down by ribs 60 in the vicinity of the air inlet 52. In this way, a space, i.e., a buffer chamber 61, is formed between the absorber 54a and the sidewall having the ink outlet 53.
When first using the ink tank 51, the ink surface 66 in the absorption chamber 55 is above the upper edge of the communication hole 59, as illustrated in FIG. 16. As the ink inside the absorbers 4a and 4b is consumed by the recording head, not shown in the drawing, the ink surface 66 is lowered. When the ink surface 66 reaches a gas-liquid exchange point 67 at the upper edge of the communication hole 59, as illustrated in FIG. 17, the ink 56 at the communication hole 59 is sucked into the absorbers 54a and 54b by capillary force. To compensate for the volume of ink sucked into the absorbers 54a and 54b, air is sent through the communication hole 59 to the ink chamber 57. Accordingly, gas-liquid exchange is carried out. During gas-liquid exchange, a flow of air, indicated by an arrow FA in the drawing, and a flow of ink, indicated by an arrow FI in the drawing, are generated.
In this way, when ink is consumed by the recording head, a volume of ink equivalent to the volume of consumed ink is supplied to the absorbers 54a and 54b, and the absorbers 54a and 54b maintain a uniform volume of ink (i.e., the level of the ink surface 66 is maintained at a predetermined height). Accordingly, the absorbers 54a and 54b maintain negative pressure in the ink tank 51 in respect to the recording head. In this way, the recording head is capable of carrying out sufficient discharge of ink.
According to the structure of the ink tank 51 illustrated in FIGS. 16 and 17, the two absorbers 54a and 54b maintain the height of the level of the ink surface 66 within the absorbers 54a and 54b and prevent air from entering the ink chamber 57 when the recording head is not used. In this way, gas-liquid exchange is carried out stably.
Such a small ink tank capable of using ink in a highly efficient manner has been commercialized by the assignee and is currently in practical use.
Japanese Patent Laid-Open No. 8-20115 (corresponding to U.S. Pat. No. 6,137,512) discloses an ink tank including absorbers made of thermal-plastic olefin-based resin. The ink tank has excellent storage stability and can be easily recycled since the chassis of the ink tank and the fiber absorber are made of similar materials.
Japanese Patent Laid-Open No. 10-24603 discloses a structure in which a groove for letting gas in from an absorption chamber to an ink changer is disposed on the bottom surface of an ink tank. The groove is provided to let gas in from an absorption chamber during gas-liquid exchange carried out when the ink surface at the inner portion of an absorber, which is where capillary force is weakest, is lowered.
Recently, the recording speed of known inkjet recording apparatuses has been increased. Along with the increase in recording speed, the inkjet recording apparatus is required to increase the volume of ink supplied per unit time from the ink tank to the recording head. If the volume of ink supplied from the ink tank is increased in the above-described ink tank, the air supply to the ink chamber or the ink supply from the ink chamber will not be able keep up with the volume of ink being consumed from the ink tank. Therefore, the level of the ink surface in the absorption chamber 55 will be lowered. As illustrated in FIG. 18, if the level of the ink surface is significantly lowered inside the absorption chamber 55, a sufficient amount of ink cannot be held in the vicinity of the ink outlet 53 even though a substantial volume of ink 56 remains inside the ink chamber 57. This lowering of the level of the ink surface must be prevented because it might result in an interruption of the ink supply to the recording head.
To increase the recording speed and improve the quality of the recorded image of known inkjet recording apparatuses, the length of the recording heads and the density of the ink discharger have been increased. When filling ink into an inkjet recording head or when operating the inkjet recording head to carry out recording, the discharge ability of the recording head is recovered by removing thickened ink from the vicinity of the ink dischargers. The removal of the thickened ink is carried out by sucking out the ink from the ink dischargers. If the length of the recording heads and the density of the ink discharger are increased, the suction action must be carried out at a higher pressure. At this time, the level of the ink surface must be prevented from lowering inside the absorption chamber. If the level of the ink surface is lowered during suction, the meniscus breaks at the surface where the absorber and the sidewall of the ink tank come into contact. As a result, air bubbles may be sucked into the recording head. Even if the meniscus does not break, the ink supply may fail.
In this way, to prevent the level of the ink surface from being lowered in the absorption chamber even when the volume of ink being supplied is increased, the distance between the ink outlet and the communication hole can be reduced, i.e., the flow resistance in the area is reduced, to improve the ability of supplying ink. However, if the distance between the ink outlet and the communication hole is reduced, for example, ink leakage may occur when the ink chamber is disposed above the absorption chamber and left untouched. In other words, the reliability of the ink tank may be reduced. The flow resistance may be reduced by reducing the density of the absorbers. However, also in such a case, the ink-holding power of the absorber is reduced and the reliability of the ink tank may be reduced.